Lately, voice, video, and broadcasting services have been provided to users using ultrahigh frequency (UHF), such as a digital television (DTV) service, a terrestrial digital multimedia broadcasting (T-DMB) service, a digital video broadcasting-handheld (DVB-H) service, a satellite digital multimedia broadcasting (S-DMB) service, and a digital audio broadcasting (DAB) service. The wavelength of a usable frequency bandwidth of the services is greater than the size of a mobile phone.
In order to receive such services, an antenna needs to have a bigger size which may be bigger than the mobile phone. Such a bigger antenna makes a user feel inconvenience in using the mobile phone and it is difficult to design the mobile phone to internally include such an antenna.
Since it is necessary to design a mobile terminal in consideration of mobility and portability as well as a multimedia function, various technologies have been introduced for internally installing an antenna in a case of a mobile terminal. An internal antenna has been widely used for cellular mobile communication, PCS mobile communication, wireless local area network (W-LAN) because a wavelength of a usable frequency bandwidth thereof is shorter than a case of a terminal. However, it is impossible to use the internal antenna if the wavelength of a usable frequency is greater than a case of a terminal, for example, DVB-H or T-DMB.
A mobile phone antenna is generally classified into a monopole antenna and a non-monopole antenna.
The monopole antenna is an antenna inducing resonance by reducing the size of an antenna from a ½ wavelength to a ¼ wavelength using an image effect of a ground plane. For example, a whip antenna, a helical antenna, a sleeve antenna, and an N-shaped antenna are the monopole antenna. Most of the monopole antennas are an external antenna and has a ¼ wavelength.
In case of a monopole antenna, the size thereof was reduced by installing a disk shaped top loaded at an end of an antenna element, twisting an antenna element in a meander shape, or rolling up an antenna element like a helical antenna. However, it was difficult to maintain the size of an antenna smaller than a 1/10 wavelength while maintaining high antenna efficiency.
Also, a disk shaped monopole antenna having an inductance element such as a helical antenna was introduced to reduce the size thereof. Although the disk shaped monopole antenna has a wideband characteristic, the disk shaped monopole antenna has a complicated structure, a high height, and a wide width. Thus, it was difficult to internally install the disk shaped monopole antenna in a case of a terminal.
As an non-monopole antenna, an inverted-F antenna, a planar inverted F-antenna, a diversity antenna, a micro-strip patch antenna, an electronic identification (EID) antenna, a full-short circuit planar inverted-F antenna (FS-PIFA), a radiation-coupled dual-L antenna (RCDLA), and a double-T slot antenna (DTSA) antenna were introduced.
Here, the planar inverted-E antenna, the micro-strip patch antenna, and a dielectric antenna were an internal antenna according to a related art. That is, the size of the internal antenna is reduced using a dielectric substance or by reducing an electric length thereof through deforming a shape of an antenna element. However, it was difficult to maintain the omni-directional radiation pattern of vertical polarization due to a printed circuit board (PCB) vertically disposed in a mobile phone because the internal antenna was disposed at the PCB only.
Furthermore, a loop antenna having a cap capacitor was introduced to reduce the size thereof. However, it was difficult to maintain high antenna efficiency because the loop antenna having the cap capacitor does not use the resonance characteristics of antenna element.
Meanwhile, since a small antenna physically occupies a small space, the bandwidth of a small antenna is limited in order to maintain good antenna efficiency. Here, the antenna efficiency is a ratio between power radiated from an antenna and power supplied to an antenna.
Therefore, it is limited to use a small antenna for a T-DMB phone, a DVB-H phone, a UHF band terminal, a T-DMB cellular phone, a T-DMB PCS phone, and a DVB-H GSM phone, which provide related services using various frequency bands.
That is, there have been demands for developing a small antenna capable of ultra-wide band transceiving by inducing a plurality of resonant frequencies.
FIG. 1 illustrates a loop antenna in accordance with a related art.
As shown in FIG. 1, the loop antenna according to the related art includes an antenna element 101, a printed circuit board (PCB) 103, a terminal case 105, and a ground plane 107. Here, the antenna element denotes an element having an electric length that decides a resonant frequency. In FIG. 1, an antenna line is used as an antenna element. The PCB 103 includes general circuits such as a RF element connected with an amplifier, a mixer, or an analog-to-digital (AD) converter.
In general, a self-resonance having a maximum valid area against a wavelength is induced when an overall length of the antenna element 101 is about 1 wavelength. Here, the self-resonance denotes a resonance that is induced by the inductance of an inductor and the parasitic capacitance component. The inductor functions as the capacitor by the parasitic component of the inductor at a frequency greater than the self-resonant frequency. Therefore, the inductance component greatly changes in the self-resonance.
However, the loop antenna according to the related art has a similar size of a ¼ wavelength monopole antenna if the self-resonant frequency of 1 wavelength is used as a usable frequency. Thus, the loop antenna according to the related art was not a small antenna. Also, the loop antenna according to the related art has a limitation to control the self-resonant frequency because the length of an antenna element is fixed by a case thereof. For example, a loop antenna is internally fixed at the terminal case 105. If it is required to change the length of the antenna element 101 for controlling the self-resonant frequency, it is also required to change the terminal case 105.
Since a size and a shape of a mobile communication terminal are limited due to the portability, a terminal case is designed and produced at first in consideration of the preference of a consumer and the convenience of a user. Therefore, an antenna must be designed in consideration of the type and shape of a terminal case as well as impedance matching and self-resonant frequency control. That is, an antenna must be capable of controlling a self-resonant frequency regardless of the size of a terminal case.
However, it is difficult to control a resonant frequency without changing the length of a loop antenna because the loop antenna is generally disposed inside a terminal case. Therefore, there have been demands for developing a loop antenna capable of controlling a resonant frequency without changing the overall length of an antenna element by setting a section of an antenna line and changing the antenna line section to control the resonant frequency.
As described above, there have been also demands for developing a loop antenna having high antenna efficiency, having ultra wide bandwidth receiving characteristics by inducing a plurality of resonance frequencies, and having a small size.